| As the economy and industry develop rapidly,the toxic heavy metals that had existed in the environment for a long time which had been a potential hidden danger to the ecological environment.For example,when Lead(Pb),Cadmium(Cd)and Copper(Cu)entered the environment,they were not only difficult to be degraded and treated,but also could be accumulated in living organisms through the transmission of food chain,causing serious health hazards to the public.Fly ash(FA),being industrial waste products of coal-fired power plants,were produced in huge quantities annually.However,it had a good application potential for heavy metal removal due to its smaller fineness and good pore structure.Different modification methods had been used to improve the adsorption performance of FA,but most of them had large energy consumption,high preparation cost and complicated operation,so it was important and practical to optimize the modification technologies.In this study,using KOH as the activator,FA was modified by a one-step low-temperature melting method to prepare KOH-modified fly ash(KFA)with high adsorption performance with the aim of removing Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)out from wastewater.The optimal modification conditions were determined by continuously optimizing the preparation processes.The changes in physicochemical properties before and after FA modification would be clarified by using various characterization means.The adsorption performance on Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)by KFA under the influence of different environmental factors was investigated by performing single system and ternary system adsorption experiments.The adsorption behavior of KFA on the three metal ions was analyzed by combining adsorption isotherm and adsorption kinetics models,and further revealed the adsorption mechanism.The cycle regeneration performance of KFA was explored by adsorption-desorption experiments.The main findings were as follows:(1)The effect of modified fly ash on Pb(Ⅱ)adsorption was investigated by changing the ash to alkali ratio,modification time and modification temperature.The qualities of KOH were too little or too much to improve the adsorption capacity of modified fly ash,and the different ratios of ash to alkali on the adsorption capacity of modified fly ash for Pb(Ⅱ)followed 5:7>5:8≈5:10>5:5.When the modification times were too short or too long,it would influence the removal effect of Pb(Ⅱ)by modified fly ash,and the Pb(Ⅱ)adsorption amounts by modified fly ash under different modification times were in the order of 2.5 h>3.5 h>5 h>1 h.The increase of modification temperature would make the adsorption ability of modified fly ash enhanced,but too high temperature would inhibit the removal of metal ions,and the adsorption capacity of modified fly ash for Pb(Ⅱ)at different modification temperatures followed300°C>250°C>350°C>150°C.By comparison,the optimal KOH-modified fly ash preparation parameters were 5:7,300°C,2.5 h.(2)The differences in the physicochemical properties of FA and KFA were investigated using various characterization methods.After the determination of scanning electron microscopy(SEM),energy dispersive X-ray(EDS),Brunauer–Emmett–Teller(BET),X-ray diffraction(XRD),Fourier transform infrared spectroscopy(FTIR)and Zeta potential,it was found that after KOH modification,the surface of KFA was rough and porous,with an increase in specific surface area of 21.77 m~2/g.Both the crystal structures of the original mineral components,mullite and quartz,were destroyed.And the characteristic peaks of surface functional groups changed in position and intensity.The Zero potential of KFA was 7.64.(3)The KFA adsorption capacities of Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)in single and ternary systems were analyzed by batch adsorption experiments.The single system adsorption results showed that,at 25°C,KFA dosage of 1 g/L,initial solution concentration of 100 mg/L and pH of 5,Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)adsorptions reached equilibrium at 10,240 and 120 min,with equilibrium adsorption amounts of 98.58,95.22 and 68.44 mg/g.Meanwhile,with the same experimental conditions in the ternary system,the balanced adsorption capacities of Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)decreased to 93.37,22.30 and 27.87 mg/g,and the equilibrium adsorption times changed to 180,30 and 30 min,respectively.In addition,the coexisting cations could hinder the adsorption of the three metal ions by KFA,with Ca(Ⅱ)having the most obvious inhibitory effect.The cyclic regeneration study showed that KFA still had good adsorption capacity for the three metal ions after five adsorption-desorption experiments.(4)The adsorption mechanisms of KFA on Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)were explored by model fitting and characterization.The results show that the Pseudo-second order kinetic model provided a better fit for the single and ternary adsorption processes of KFA on Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ),which indicated that the adsorption process were dominated by chemisorption and proceeded as a homogeneous monomolecular layer.The single adsorption of Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)by KFA showed a better fit to the Langmuir model,indicating that the adsorption process was endothermic.Combined with the characterization analysis,it was concluded that the adsorption mechanisms of KFA on Pb(Ⅱ),Cd(Ⅱ)and Cu(Ⅱ)include ion exchange,complexation and electrostatic attraction.In summary,the KFA which is prepared by the optimized preparation process has the advantages of low energy consumption and low cost.Not only the physicochemical properties are changed,but also the high adsorption capacity is achieved,which can realize heavy metal ion removal in a single and simultaneous manner.And there is good potential for the development of KFA in recycling.In conclusion,this work can provide valuable theoretical basis and technical reference for the practical application of removing heavy metal ions from the aqueous environment. |
PDF Full Text Request